System and method for detecting adverse atmospheric conditions ahead of an aircraft

ABSTRACT

System and method for detecting adverse atmospheric conditions ahead of an aircraft. The system has multiple, infrared cameras  8  adjusted to spatially detect infrared radiance in different bands of infrared light, wherein each camera is connected to an image processing computer that processes and combines the images, and generates video display signals for producing a video display which indicates the position of the adverse atmospheric conditions relative to the aircraft. Each of the cameras is provided with a respective filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions. The image processing computer is adapted to identify adverse atmospheric conditions, said identifying being based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude of the aircraft. The image processing computer is further adapted to display the identified adverse atmospheric conditions as a spatial image on a display.

The present invention relates to a system and method for detectingadverse atmospheric conditions ahead of an aircraft. The system has aplurality of infrared cameras that may detect, for example, sulphurdioxide and particles such as volcanic ash, wind-blown dust and iceparticles. It also comprises a computer that processes the images and adisplay to show the crew of the aircraft the adverse conditions.

There are a number of adverse atmospheric conditions that are desirableto detect. These include volcanic ash, toxic gases such as sulphurdioxide gas, wind-blown dust and ice particles.

Volcanic clouds contain silicate ash and gases that are hazardous toaviation. Several encounters between jet aircraft and volcanic ash haveresulted in significant damage due to ingestion of ash into the hotparts of the engine, subsequent melting and fusing onto the turbineblades. Ash can also block the pitot static tubes and affect sensitiveaircraft instruments, as well as abrade the leading edges of parts ofthe airframe structure. Volcanic gases, principally SO₂ are lessdangerous to aircraft, but detection of SO₂ can be used as an indicatorof volcanic ash as these substances are often collocated and aretransported together by atmospheric winds. Another important gas involcanic clouds is water vapour (H₂O gas). Water vapour occurs incopious amounts in volcanic clouds either through entrainment of ambientair or from water from the volcanic source (e.g. sea water is a commonsource for volcanoes on islands or in coastal regions). Once in theatmosphere, the water vapour can condense on ash nuclei rapidly formingice with a much smaller radius than ice in normal meteorological clouds.These abundant, small-sized ice particles are hazardous to aircraftbecause the rapid melting of the ice when in contact with the hotengines, releases the ash nuclei which then fuses onto the turbineblades, affecting the engine performance and potentially causing theengine to stall.

Damage to aircraft can be counted in the millions of dollars. Mostserious aircraft encounters with ash clouds have been at cruisealtitudes, but there is also a hazard to aircraft at airports affectedby volcanic ash. These airports are usually close to an active volcanobut they can also be at some distance from the source of the eruptiondue to atmospheric transport that brings ash into the region.

The cost of ash hazards to airport operations is not known, but must besignificant if the costs include those due to delays to landings andtake-offs as well as re-routing costs incurred by airline operators. Therecent (14 Apr., 2010) eruption of Eyjafjallajoekull in Iceland isestimated to have cost the airline industry approximately US$2 bn.Currently there are no regulatory requirements for airport operators toprovide warnings of ash hazards. Warnings are issued based oninformation from volcano observatories, meteorological advisories and,in some cases, radar observations of eruption columns. Radar informationis generally only reliable at the start of an eruption when the ashcloud is thick and usually such information is only available atairports in close proximity to an erupting volcano. For airports distantfrom the source of ash there are few direct observations available. Someobservations come from satellite systems and other sources ofinformation come from trajectory forecasts based on wind data and cloudheight information. Much of this information is sporadic and untimelyand there is a need for better detection systems.

Other adverse atmospheric conditions include the toxic gases emitted byvolcanoes and industrial plants. Of particular importance and abundanceis sulphur dioxide (SO₂) gas.

SO₂ clouds from volcanoes will react with water vapour in the atmosphereto produce sulphuric acid which can damage aircraft. It will beappreciated that the sulphur dioxide may be found in areas separate fromthe volcanic ash. An aircraft can fly through sulphur dioxide withoutpassing through ash. Post encounter treatment of the engine in the caseof sulphur dioxide encounter would be different to and considerablycheaper than the equivalent treatment required of an engine during anash encounter. Accordingly, it would be desirable to be able to warnaircraft of SO₂ clouds.

Ash and other particles can under the right conditions initiate iceparticle formation when water freezes around these cores. Accordingly,wind-blown dust and ice particles can be a significant hazard toaircraft, vehicles and the like.

Jet aircraft at cruise altitudes (above 15,000 feet), travel rapidly(>500 km hr⁻¹) and currently do not have a means for detecting volcaniccloud hazards ahead. Because of the high speed, a detection method mustbe able to gather information rapidly and provide an automated alert andspecies identification algorithm, capable of distinguishing volcanicsubstances from other substances in the atmosphere (e.g. meteorologicalclouds of water and ice).

WO2005031321A1, WO2005068977A1 and WO2005031323A1 teach methods andapparatus for monitoring of sulphur dioxide, volcanic ash and wind-blowndust, using at least two wavelengths of infrared radiation correspondingto an adverse atmospheric condition.

U.S. Pat. No. 3,931,462 teaches the use of an UV video system formeasuring SO₂ in plume from a smokestack.

U.S. Pat. No. 4,965,572 teaches methods and apparatus for detecting lowlevel wind shear type turbulence remotely, such as by an infraredtemperature detector.

U.S. Pat. No. 5,140,416 discloses a system and method for fusing ormerging video imagery from multiple sources such that the resultantimage has improved information content. The sensors are responsive todifferent types of spectral content in the scene being scanned, such asshort and long wavelength infrared.

U.S. Pat. No. 5,654,700 and U.S. Pat. No. 5,602,543 teach an adverseatmospheric condition detection system for aircraft that monitorsconditions ahead of aircraft using infrared detectors, displays theposition, warns and reroutes aircraft.

According to a first aspect, the present invention provides a system fordetecting adverse atmospheric conditions ahead of an aircraft, includinga plurality of infrared cameras mounted on the aircraft, wherein: theinfrared cameras are adjusted to spatially detect infrared radiance indifferent bands of infrared light, each camera is connected to an imageprocessing computer that processes and combines the images, andgenerates video display signals for producing a video display whichindicates the position of the adverse atmospheric conditions relative tothe aircraft; each of the cameras is provided with a respective filteradjusted to filter infrared light with a bandwidth corresponding toinfrared bandwidth characteristics of an adverse atmospheric conditionfrom a set of adverse atmospheric conditions; the image processingcomputer is adapted to identify adverse atmospheric conditions, saididentifying being based on threshold conditions and using the detectedinfrared radiance, data from a look-up table and measured parametersincluding information on the position and/or attitude of the aircraft;and the image processing computer is further adapted to display theidentified adverse atmospheric conditions as a spatial image on adisplay.

The present invention is advantageous in that it provides an apparatussuited for aircraft that detects adverse atmospheric conditions, inparticular caused by volcanoes, and visualizes them for the crew of theaircraft. The invention is particularly useful for detecting volcanicclouds. For example, the present invention can enable the rapiddetection of volcanic substances ahead of a jet aircraft at cruisealtitudes and the simultaneous detection and discrimination of volcanicash, SO₂ gas and ice-coated ash particles. Preferably, the inventionprovides algorithms and processes for converting raw camera data toidentify ash, SO₂ gas and ice coated ash.

The system preferably monitors the field of view of the aircraft.

The cameras of the invention may be uncooled microbolometer collocatedcameras.

In one embodiment of the system the pitch angle and ambient temperatureare accounted for in the look-up table.

The adverse atmospheric conditions preferably include volcanic ash, icecoated ash, water vapour and sulphur dioxide. The measured parameterscan include pitch angle and ambient temperature.

Preferably, the threshold conditions are pre-computed using a radiativetransfer model of the atmosphere.

Preferably, the image processing computer is arranged to determinebrightness temperatures from the detected infrared radiance, and saididentifying includes determining whether values related to thebrightness temperatures meet the threshold conditions.

The system may also include one or more external blackened shuttersagainst which the imaging cameras are pre-calibrated for providingin-flight calibration values.

Most preferably, the system provides a statistical alert based onanalysis of images determined to show an adverse condition of ash,sulphur dioxide or ice-coated ash. The statistical alert uses spatialand temporal information and can be tuned according to in-flight teststo reduce false-alarms and ensure robustness.

For these embodiments there can be a computer program loadable into theinternal memory of a processing unit in a computer based system,comprising software code portions for performing the said steps.

For these embodiments there can be a computer program product stored ona computer readable medium, comprising a readable program for causing aprocessing unit in a computer based system, to control an executionaccording to the said steps.

Preferably the system is arranged to detect at least the three volcanicsubstances (ash, SO₂ and ash coated ice particles) in the air ahead ofthe aircraft by a remote method, and in addition be capable ofdiscriminating these from other meteorological clouds of water dropletsand ice.

The invention also more generally provides a system for detectingadverse atmospheric conditions ahead of an aircraft, including aplurality of infrared cameras mounted on the aircraft, wherein: theinfrared cameras are adjusted to spatially detect infrared radiance indifferent bands of infrared light; each camera is connected to an imageprocessing computer that processes and combines the images, wherein eachof the cameras is provided with a respective filter adjusted to filterinfrared light with a bandwidth corresponding to infrared bandwidthcharacteristics of an adverse atmospheric condition from a set ofadverse atmospheric conditions; and the image processing computer isadapted to identify and display adverse atmospheric conditions, saididentifying being based on threshold conditions and using the detectedinfrared radiance and measured parameters including information on theposition and/or attitude of the aircraft.

According to another aspect, the present invention provides a method fordetecting adverse atmospheric conditions ahead of an aircraft anddisplaying said adverse atmospheric conditions, comprising spatiallydetecting infrared radiance in different bands of infrared light using aplurality of infrared cameras; and, for each camera: i) Filtering theinfrared radiation with a filter adjusted to filter infrared light witha bandwidth corresponding to infrared bandwidth characteristics of anadverse atmospheric condition in a set of adverse atmosphericconditions; ii) identifying likely occurrences of adverse atmosphericconditions based on threshold conditions and using the detected infraredradiance, data from a look-up table and measured parameters includinginformation on the position and/or attitude of the aircraft; and iii)processing the identified likely occurrences of adverse atmosphericconditions to create a spatial image.

In one embodiment, the method further comprises the step of iv)combining the image with images from other cameras and information onthe aircraft flight path.

The adverse atmospheric conditions preferably include volcanic ash, icecoated ash, water vapour and sulphur dioxide. The measured parameterscan include pitch angle and ambient temperature.

In a further aspect, the invention provides a system for detectingvolcanic clouds ahead of an aircraft, including one or more infraredcameras mounted on the aircraft, the infrared cameras are adjusted tospatially detect infrared radiance in different bands of infrared light,each camera is connected to an image processing computer that processand combines the images, combining them with flight path informationfrom the aircraft and generates video display signals for producing avideo display which indicates the position of the adverse conditionsrelative to the aircraft; characterized in that each of the cameras isprovided with a respective filter adjusted to filter infrared light witha bandwidth corresponding to infrared bandwidth characteristics of oneof the volcanic species in a set of volcanic species, and that the imageprocessing computer is adapted to identify and display species as aspatial image on a display by means of threshold look-up tables for therespective species mapping thresholds for the infrared radiance, abovewhich species are likely to occur, with measured parameters.

In a still further aspect, the invention provides a method for detectinga volcanic cloud ahead of an aircraft and displaying said cloud,processing information from one or more infrared cameras spatiallydetecting infrared radiance in different bands of infrared light,combining the information with flight path information from the aircraftcharacterized in the steps of for each camera:

i) Filtering the infrared radiation with a filter adjusted to filterinfrared light with a bandwidth corresponding to infrared bandwidthcharacteristics of one of the volcanic species in a set of volcanicspecies; ii) identifying likely occurrences of species by looking updetected infrared radiance values in a threshold look-up table mappingthresholds for the infrared radiance, above which species are likely tooccur, with measured parameters; iii) processing the identified likelyoccurrences of species to create a spatial image.

Preferred embodiments of the present invention will now be described byway of example only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic of a single camera with filter, lens, shutter andprotective window;

FIG. 2 is an example configuration for the multiple camera system;

FIG. 3 shows an ash cloud on the display;

FIG. 4 shows a diagram of radiative transfer calculation for ahorizontal path in a clear atmosphere for three different flightaltitudes; and

FIG. 5 shows a diagram of line strengths for the two bands of SO₂ at 8.6μm and 7.3 μm. The response functions for the filters of the system arealso shown.

The basic principle of detection of volcanic substances ahead of theaircraft relies on the use of filtered infrared radiation in the regionof 6-13 p.m. Within this region, narrow (0.5-1.0 μm) bands are selectedfor detection of ash, water vapour, SO₂ gas and ice coated ash. Thepreferred detection method is to use wide-field-of-view, rapid sampling,imaging, uncooled microbolometer cameras.

A microbolometer is used as a detector in thermal cameras. Infraredradiation strikes the detector material, heating it, and thus changingits electrical resistance. This resistance change is measured andprocessed into temperatures which can be used to create an image. Unlikeother types of infrared detecting equipment, microbolometers do notrequire cooling.

Typically this camera may contain 640×512 pixels×lines, have a noiseequivalent temperature difference of 50 mK (or better) at 300 K in the10-12 μm region, and provide sampling rates up to 60 Hz. Five collocatedcameras are envisaged for the simultaneous detection of ash, SO₂ gas,H₂O gas, and ice-coated ash. Each camera has a detector that issensitive to infrared radiation within the region 6-13 μm. Narrowbandfilters are placed over each camera to restrict the spectral content ofthe radiation for the purpose of species identification. The camerasshare the same field of view ahead of the aircraft and therefore, inprinciple, multiple, simultaneous narrowband infrared images can beacquired in real-time at sampling rates of up to 60 Hz. These collocatedimages can be rapidly processed using special algorithms to identifyeach of the four target species specified earlier.

One embodiment of the system has 5 collocated imaging cameras, but thisnumber could be more or less depending on the requirements of the user.A generic example of the camera in the system is shown in FIG. 1.Infrared radiation from ahead of the aircraft enters the filter 1 ofeach camera and is focused through the camera lens 2 and falls on thedetector array 3. The shutter 4 is used for calibration (see below). Thesignals are transferred via a standard high-speed communication protocol5 to a computer for further processing. To protect the filter and lenswhile the system is viewing ahead of the aircraft, an IR transparentwindow 7 (e.g. Germanium glass) is attached between the shutter andfilter. The shutter is temperature controlled 6 and blackened on theside facing the optics.

An example configuration for the multiple camera system is shown in FIG.2 with five cameras 8. The protective shutter 4 may be mechanicallydriven in front of the assembly 9 and withdrawn when the system is inuse. The germanium glass window 7 provides protection from debris, whilein viewing mode. The signal 5 and power 10 lines are at the back of theassembly housing 9, which houses electronics, frame grabber and computerhardware. Five cameras are shown, but the configuration could consist ofmore or less cameras depending on the number of hazards to beidentified. For example a system with two cameras would permitidentification of volcanic ash and ice-coated ash.

The cameras are pre-calibrated prior to installation on the aircraft sothat each camera registers the same digital signal when exposed to thesame amount of infrared radiation. This can be achieved by pointing eachcamera, without its filter, at a known source of infrared radiation(known constant temperature) and recording the digital signal from eachpixel of each camera. A look-up table can be determined by varying thesource temperature through the range 210 to 300 K, in steps of 10 K (forexample) for each camera, giving a table of 640×512×10×2 values,assuming a linear calibration. This process can be repeated for eachnarrowband filter used. Once on board the aircraft, intermittentre-calibrations can be performed by inserting a heated and blackenedshutter in front of the filter and recording the digital countscorresponding to the known (controlled) temperature of the shutter. Theshutter also serves the dual purpose of providing protection againstdebris and dirt directed toward the camera during take-off and landing,when the system of the present invention is deactivated. It will beunderstood that, optionally, a second shutter could be used to provide asecond calibration point in a linear calibration equation. The use of asecond shutter is simply a matter of practical convenience and does notalter the main operating principle of the invention.

The system is activated once the aircraft has reached cruise altitudeand whenever an airborne hazard is detected and the aircraft conductsevasive manoeuvres by altering direction-flight altitude and heading. Indeactivated mode the shutter is closed. Before activation apre-calibration cycle for the system (all 5 cameras) is conducted. Theshutter is opened and the system begins to collect images. Commercialcameras can sample as fast as 60 Hz and this is the preferred samplingrate (or higher). However, some export restrictions apply to somecameras and this means lower sampling rates may apply. In the discussionthat follows we assume a sampling rate of 8 Hz, as at this frequencythere are no export restrictions. The basic principle is unchanged whenusing a higher sampling frequency.

Each camera provides 8 images of size N pixels by M lines every second.The look-up table is used together with the on board calibration data toconvert the digital signals to a brightness temperature (BT_(i,j,k)),where k represents the camera number and k=1, 2, 3, 4 or 5, in thecurrent system, and i and j are pixel and line numbers, respectively.The brightness temperature is determined from:

$R_{i,j,k} = \frac{c_{1}v_{k}^{3}}{^{c_{2}{v_{k}/{BT}_{i,j,k}}} - 1}$

Where:

R_(i,j,k) is the radiance at pixel i, line j and filter k

v_(k) is the central wavenumber for camera filter k

BT_(i,j,k) is the brightness temperature

c₁ and c₂ are the Einstein radiation constants

The radiance R_(i,j,k) is determined from the pre- and post-calibrationprocedures and is assumed to be a linear function of the digital signalcounts. Camera images may be averaged in order to reduce noise andimprove the signal-to-noise ratio of the system.

For illustration purposes only, we shall concentrate on one image pixeland assume that all other pixels can be treated in the same manner,noting that the calibration look-up table is different for every pixel.Then, the data for one pixel consists of the measurements: BT1, BT2,BT3, BT4 and BT5, where these represent brightness temperatures fromeach of the five cameras (e.g BT1 is the brightness temperature for thatpixel in camera 1 which has filter 1).

The system of the present invention is linked into the aircraftinstrument data stream so that GPS coordinates, altitude (z), longitude(l), latitude (q), heading (h), direction (d), roll (r), yaw (y), pitch(x), time (t), speed over the ground (v), wind speed (w) and ambienttemperature (Ta) are available at a sampling rate of at least 1 andpreferably faster.

In an embodiment the system uses filters at the following centralwavenumbers (in cm⁻¹):

TABLE 1 Filter specifications for an embodiment of the presentinvention. Central wavenumber Bandwidth Filter (cm⁻¹) (cm⁻¹) NEDT (mK)Purpose 1 1410 100 200 H2O 2 1363 100 200 SO2 3 1155 100 200 SO2/ash 4929 60 100 Ash/ice 5 830 60 100 Ash/ice

Ash Detection Algorithm

A pixel is declared to be ash if the following conditions are met ateach instance:

DT1_(Ash)=(BT4−BT5)/Ta>T1_(Ash)(Ta,r,y,x)/Ta  (1)

DT2_(Ash)=(BT3−BT5)/Ta>T2_(Ash)(Ta,r,y,x)/Ta  (2)

Where T1_(Ash) and T2_(Ash) are temperature differences determined frompre-computed radiative transfer calculations for a set of parameters,including ambient temperature (Ta) and realistic aircraft roll, pitchand yaw values. Note that DT1_(Ash) and DT2_(Ash) are non-dimensionalquantities and are strictly indices.

An alert is sounded if a sequence of 8 consecutive occurrences ofcondition (1) and (2) happen for a pre-defined fraction of the totalimage. A value of 5% of the total number of pixels in the differenceimage is used, but this can be tuned as necessary a lower value set ifthe aircraft is operating in airspace declared, or likely to beinfluenced by volcanic ash; a higher value in unaffected areas.

H₂O Detection Algorithm

A pixel is declared to be water vapour affected if the followingconditions are met at each instance:

DT _(wv) =BT1−Ta>T _(wv)(Ta,r,y,x)  (3)

Where T_(wv) is a temperature difference determined from pre-computedradiative transfer calculations for a set of parameters, includingambient temperature (Ta), and realistic aircraft roll, pitch and yawvalues.

No alert is sounded, but T_(wv) is used with the ice algorithm if thatalert is sounded.

Ice-Coated Ash (ICA) Detection Algorithm

A pixel is declared to be ICA if the following conditions are met ateach instance,

DT _(ICA)=(BT4−BT5)/Ta<T _(ICA)(Ta,r,y,x)/Ta  (4)

Where T_(ICA) is a temperature difference determined from pre-computedradiative transfer calculations for a set of parameters, includingambient temperature (Ta), and realistic aircraft roll, pitch and yawvalues.

An alert is sounded if a sequence of 8 consecutive occurrences ofcondition (4) happen for a pre-defined fraction of the total image. Avalue of 5% of the total number of pixels in the difference image isused, but this can be tuned as necessary a lower value set if theaircraft is operating in airspace declared or likely to be influenced byvolcanic ash; a higher value in unaffected areas. When the alert issounded condition (3) is checked and if this condition is met, the pixelis confirmed to be ICA. The use of the water vapour condition isentirely novel and reduces the false alarm rate for detecting hazardoussmall-sized ice-coated ash particles.

SO₂ Detection Algorithm

A pixel is declared to be SO₂ if the following conditions are met ateach instance,

DT1_(SO2)=(BT1−BT2)/Ta<T1_(SO2)(Ta,r,y,x)/Ta  (5)

DT2_(SO2)=(BT3−BT5)/Ta<T2_(SO2)(Ta,r,y,x)/Ta  (6)

Where T1_(SO2) and T2_(SO2) are temperatures determined frompre-computed radiative transfer calculations for a set of parametersincluding ambient temperature (Ta), and realistic aircraft roll, pitchand yaw values.

An alert is sounded if a sequence of 8 consecutive occurrences ofconditions (5) and (6) happen for a pre-defined fraction of the totalimage. A value of 5% of the total number of pixels in the differenceimage is used, but this can be tuned as necessary—a lower value set ifthe aircraft is operating in airspace declared to be or likely to beinfluenced by volcanic ash; a higher value is used in unaffected areas.

An example of the display shown to the crew for the detection of an ashcloud is shown in FIG. 3. This is based on an ash cloud composed ofsilicate material and shows the DT1_(Ash) signal for 6 frames separatedby a constant short time difference from two cameras imaging ahead ofthe aircraft. Highest concentrations of ash are indicated in red (ordark in FIG. 3 that is in black and white); the background sky is shownin light purple (or light grey in FIG. 3). As the aircraft approachesthe hazard, the pilot can alter the heading of the aircraft to avoid it.

An important part of this invention is the use of pre-computed thresholdvalues from a detailed radiative transfer model of the atmosphere, withand without a volcanic cloud and utilizing geometrical considerationsappropriate for viewing in the infrared region (6-13 μm) from anaircraft. FIG. 4 shows a horizontal path simulation of the radiance ofthe clear atmosphere from 700-1600 cm⁻¹ at three different flightaltitudes. At 9.5 km the atmosphere appears very cold—the equivalentblackbody temperature of the horizontal path is about 227 K. Anyvolcanic cloud placed between the aircraft and the cold background willalter the radiance received by the system in a known way. The spectralcontent of the radiation contains signatures of ash, SO₂, H₂O andice-coated ash particles. These signatures can also be simulated by theradiative transfer model and the results stored in a large look-uptable. Notice that the radiance curves change with altitude and hencewith ambient temperature—the ambient temperature is determined by the onboard aircraft instrumentation and used by the detection algorithm. Itcould equally use the height (flight altitude) instead of thetemperature, but the temperature is a more robust measure.

The ash signal in these spectra is characterised by a higher brightnesstemperature in filter 4 (BT4) than in F5 (BT5), when viewing a coldbackground. The threshold values are determined by using refractiveindex data for silicates and scattering calculations are based onmeasured particle size distribution for particle with radii in the range1-20 μm according to the art. Generally, the instrument would look inthe horizontal or slightly upwards (aircraft usually have a 3° pitchangle upwards). However, the aircraft may pitch downwards, in which casethe background temperature might change from a cold background to a warmbackground. In this case, the ash signature is identified by BT4<BT5.The look-up table is constructed in such a way that the pitch angle andambient temperature are accounted for. Additionally, the roll and yawangles are compensated for, although these have only a minor influenceon the detection algorithm. Extra fail-safe thresholds are alsoincorporated into the detection algorithm by utilizing a filter near 8.6μm that has sensitivity to volcanic ash.

The operation of the ice-coated ash algorithm is similar to the ashalgorithm, except the threshold look-up table is now determined usingdata for ice (refractive indices and scattering data for smallparticles, radii<30 μm). In the case of small ice particles, BT4<BT5 forviewing into a cold background (the opposite to ash without an icecoating). Background conditions are accounted for in a similar way tothat used for the ash detection.

Normalisation of the temperature differences is done to provide somerobustness and to make the detection independent of the ambient airtemperature.

SO₂ and H₂O look-up tables are also used. SO₂ has very strongabsorptions near to 8.6 μm and 7.3 μm as FIG. 5 illustrates. Theprinciple of detecting SO2 has been described earlier and is based onradiative transfer calculations assuming the line strengths andtransmissions applicable to the case of an atmosphere loaded with SO2.Under normal conditions SO₂ has an extremely low abundance (<10⁻³ ppm),and so detection of SO₂ using these absorptions features is veryeffective in the case of volcanic clouds ahead of an aircraft.

1. A system for detecting adverse atmospheric conditions ahead of anaircraft, comprising a plurality of infrared cameras mounted on theaircraft, wherein the infrared cameras are adjusted to spatially detectinfrared radiance in different bands of infrared light; each camera isconnected to an image processing computer that processes and combinesthe images and generates video display signals for producing a videodisplay which indicates the position of the adverse atmosphericconditions relative to the aircraft; each of the cameras is providedwith a respective filter adjusted to filter infrared light with abandwidth corresponding to infrared bandwidth characteristics of anadverse atmospheric condition from a set of adverse atmosphericconditions; the image processing computer is adapted to identify adverseatmospheric conditions, said identifying being based on thresholdconditions and using the detected infrared radiance, data from a look-uptable and measured parameters including information on the positionand/or attitude of the aircraft; and the image processing computer isfurther adapted to display the identified adverse atmospheric conditionsas a spatial image on the display.
 2. The system of claim 1 wherein theset of adverse atmospheric conditions includes volcanic ash, ice coatedash, water vapour and sulphur dioxide.
 3. The system claim 2, whereinthe system is arranged to seek to identify both ice coated ash and watervapour, and wherein the identification of water vapour is used toconfirm an identification of ice coated ash.
 4. The system of claim 1wherein the threshold conditions are pre-computed using a radiativetransfer model of the atmosphere.
 5. The system of claim 1, wherein theimage processing computer is arranged to determine brightnesstemperatures from the detected infrared radiance, and said identifyingincludes determining whether values related to the brightnesstemperatures meet the threshold conditions.
 6. The system claim 1,wherein the measured parameters include pitch angle and ambienttemperature.
 7. The system claim 1, including one or more externalblackened shutters against which said imaging cameras are pre-calibratedfor providing in-flight calibration values.
 8. A method for detectingadverse atmospheric conditions ahead of an aircraft and displaying saidadverse atmospheric conditions, comprising spatially detecting infraredradiance in different bands of infrared light using a plurality ofinfrared cameras; and, for each camera: i) Filtering the infraredradiation with a filter adjusted to filter infrared light with abandwidth corresponding to infrared bandwidth characteristics of anadverse atmospheric condition in a set of adverse atmosphericconditions; ii) identifying likely occurrences of adverse atmosphericconditions based on threshold conditions and using the detected infraredradiance, data from a look-up table and measured parameters includinginformation on the position and/or attitude of the aircraft; and iii)processing the identified likely occurrences of adverse atmosphericconditions to create a spatial image.
 9. The method of claim 8,including the additional step of iv) combining the image with imagesfrom other cameras and information on the aircraft flight path.
 10. Themethod of claim 8, wherein the set of adverse atmospheric conditionsincludes volcanic ash, ice coated ash, water vapour and sulphur dioxide.11. The method of claim 10, further comprising identifying likelyoccurrences of both ice coated ash and water vapour, wherein theidentification of water vapour is used to confirm an identification ofice coated ash.
 12. The method of claim 8, wherein the thresholdconditions are pre-computed using a radiative transfer model of theatmosphere.
 13. The method of claim 8, further comprising, for eachcamera, determining brightness temperatures from the detected infraredradiance, and wherein said identifying includes determining whethervalues related to the brightness temperatures meet the thresholdconditions.
 14. The method of claim 8, wherein the measured parametersinclude pitch angle and ambient temperature.
 15. The method of claim 8,further comprising pre-calibrating the imaging cameras against one ormore external blackened shutters, for providing in-flight calibrationvalues.
 16. (canceled)
 17. A computer program product stored on acomputer readable medium, comprising a readable program which whenexecuted by a computer causes the computer to carry out a methodcomprising spatially detecting infrared radiance in different bands ofinfrared light using a plurality of infrared cameras; and, for eachcamera: i) filtering the infrared radiation with a filter adjusted tofilter infrared light with a bandwidth corresponding to infraredbandwidth characteristics of an adverse atmospheric condition in a setof adverse atmospheric conditions; ii) identifying likely occurrences ofadverse atmospheric conditions based on threshold conditions and usingthe detected infrared radiance, data from a look-up table and measuredparameters including information on the position and/or attitude of theaircraft; and iii) processing the identified likely occurrences ofadverse atmospheric conditions to create spatial image.
 18. The computerprogram product of claim 17, wherein the method further comprisescombining the image with images from other cameras and information onthe aircraft flight path.
 19. The computer program product of claim 17,wherein the set of adverse atmospheric conditions includes volcanic ash,ice coated ash, water vapour and sulphur dioxide.
 20. The computerprogram product of claim 17, wherein the method further comprisesidentifying likely occurrences of both ice coated ash and water vapour,wherein the identification of water vapour is used to confirm anidentification of ice coated ash.
 21. The computer program product ofclaim 17, wherein the threshold conditions are pre-computed using aradiative transfer model of the atmosphere.
 22. The computer programproduct of claim 17, wherein the method further comprises, for eachcamera, determining brightness temperatures from the detected infraredradiance, and wherein said identifying includes determining whethervalues related to the brightness temperatures meet the thresholdconditions.